The present invention concerns apparatus and a method which uses microscopy to determine characteristics of materials having relatively high indices of refraction and in particular, to apparatus and a method for inspecting semiconductor wafers from the back side, using a lens which has a relatively high numerical aperture and which is either in contact with or very close to being in contact with the wafer.
As integrated circuits are designed to include more and more components, the individual components are designed to be smaller and smaller. Some components, however, such as storage capacitors for dynamic random access memory (DRAM) cells, cannot be made smaller than a defined minimum size without degrading their performance. Recently, the numbers of components of this type which may be placed on an integrated circuit has been increased through the use of three-dimensional structures such as trenches.
Currently these trenches have diameters of between one and two microns (.mu.m), depths of six to eight .mu.m and aspect ratios (depth to diameter) of eight to one. In the near future, devices having aspect ratios approaching 100 to 1 and diameters of less than one .mu.m may become feasible.
While these properties are desirable to allow relatively large numbers of capacitors to be fabricated on an integrated circuit, they make visual inspection of the capacitors almost impossible even with a powerful microscope. This is because silicon and gallium arsenide are extremely lossy at the optical frequencies which are high enough to resolve the trench structures. At present, the dominant method of inspecting components of this type is to saw the wafer so that the components may be viewed from the side using a scanning electron microscope. This technique is time consuming and it destroys the wafer. Consequently, it is difficult to determine the electrical properties of an observed structure.
Silicon and gallium arsenide are transparent to infrared radiation having wavelengths between 1.2 and 15 .mu.m Backside inspection using infrared microscopy is routinely performed for inspecting flip-chip bonding pads, making picosecond voltage measurements and various photothermal and photoacoustic measurements. Unfortunately, however, all currently available infrared microscopes are limited to numerical apertures between 0.5 and 0.8. This results in a lateral resolution of 1.5 to 2.5 .mu.m which is inadequate to resolve sub-micron trenches.
U.S. Pat. No. 4,625,114 to Bosacchi et al concerns a technique for determining the thickness of thin films through the use of frustrated total internal reflection. A hemicylindrical lens, in intimate contact with the top surface of a substrate (semiconductor wafer) upon which thin films (epitaxial layers) have been deposited, is used to couple infrared radiation into the thin film structure over a wide range of angles. The instrument determines the thickness of a layer by identifying a single angle at which frustrated total internal reflection occurs. This instrument does not form images and is insensitive to properties of the other side of substrate.
U.S. Pat. No. 4,555,767 to Case et al. relates to apparatus which measures the thickness of a uniform layer of epitaxial silicon using a Fourier transform IR spectrometer. Measured values of spectral reflectance are correlated with theoretical reflectance values to determine the actual thickness of the epitaxial layer.
U.S. Pat. No. 4,615,620 to Noguchi et al. concerns apparatus for measuring the depth of fine engraved patterns. Pits having widths of between 1 and 3 .mu.m and pitches of between 2 and 3 .mu.m may be measured using radiation which varies in wavelength from 300 nanometers (nm) to 800 nm. The apparatus is a non-contact system which irradiates the top surface of the substrate with light of varying wavelength. The measurement is based on the detection of the intensity of a diffraction ray from which the contribution of the 0th order wavelength has been excluded.
U.S. Pat. No. 3,034,398 relates to an inline infrared spectrometer which uses lenses and prisms made from germanium or silicon.